For the measurement of the properties of a moving paper sheet which is moving in a machine direction (MD), a scanner 10, as illustrated in FIG. 1 includes a pair of gauging heads 11 and 12 which scan a sheet of material such as paper 13 in a cross direction 16 perpendicular to the machine direction 14. A gap 17 is formed by the upper and lower gauging heads through which the paper 13 passes. The gauging heads themselves are mounted on a pair of spaced upper and lower beams 19 and 21 which are supported at both ends by vertical supports 22a and 22b (the latter not being shown). As is apparent from FIG. 1 the gauging heads are external to the beams 19 and 21. Internal to the beam are guides for the gauging head and electrical and cooling lines, for example, for water and air. Each beam 19 and 21 contains a slot in which the respective gauging heads 11 and 12 are mounted for cross direction scanning. To protect the interior of the beams is a dust belt 23, as illustrated in FIG. 2, which is, for example, attached to the gauging head 11 which rides along the top of the slot to partially seal it. A separate drive belt (not shown) is also provided for actually moving or scanning the gauging head.
Because of the foregoing construction, the scanner has a large cross-section and is mechanically complicated. Since it operates in an extremely hot or moist environments in many situations, cooling water or cooled air must be supplied by a separate so called power track unit which contains tubes supplying both air, cold water and electrical wires for powering the gauging heads and receiving the measurement signals. Another important factor in the measurement procedure is that the upper and lower gauging heads must move in perfect synchronism to reduce measurement errors.